A 22-year-old female with no past medical history presents to her primary care physician with a 3-day history of knee pain. She denies any recent injury or trauma. On physical examination her knee is warm, erythematous, and has diminished range of movement. The patient reports having multiple sexual partners over the last year and does not use protection regularly. Her blood pressure is 124/85 mmHg, heart rate is 76/min, and temperature is 38.3℃ (101.0℉). A joint aspiration is performed and a growth of gram-negative diplococci is noted on bacterial culture. What is the treatment of choice for this patient's condition?

Ceftriaxone monotherapy and joint aspiration

Nafcillin monotherapy and joint aspiration

A 67-year-old man presents to the physician with profuse watery diarrhea along with fever and crampy abdominal pain. He has been taking an antibiotic course of cefixime for about a week to treat a respiratory tract infection. At the doctor’s office, his pulse is 112/min, the blood pressure is 100/66 mm Hg, the respirations are 22/min, and the temperature is 38.9°C (102.0°F). His oral mucosa appears dry and his abdomen is soft with vague diffuse tenderness. A digital rectal examination is normal. Laboratory studies show: Hemoglobin 11.1 g/dL Hematocrit 33% Total leucocyte count 16,000/mm3 Serum lactate 0.9 mmol/L Serum creatinine 1.1 mg/dL What is most likely to confirm the diagnosis?

Identification of C. difficile toxin in stool

An 18-year old college freshman presents to his university clinic because he has not been feeling well for the past two weeks. He has had a persistent headache, occasional cough, and chills without rigors. The patient’s vital signs are normal and physical exam is unremarkable. His radiograph shows patchy interstitial lung infiltrates and he is diagnosed with atypical pneumonia. The patient is prescribed azithromycin and takes his medication as instructed. Despite adherence to his drug regimen, he returns to the clinic one week later because his symptoms have not improved. The organism responsible for this infection is likely resistant to azithromycin through which mechanism?

Methylation of ribosomal binding site

Decreased binding to RNA polymerase

A 67-year-old male with a past medical history of diabetes type II, obesity, and hyperlipidemia presents to the general medical clinic with bilateral hearing loss. He also reports new onset vertigo and ataxia. The symptoms started a day after undergoing an uncomplicated cholecystectomy. If a drug given prophylactically just prior to surgery has caused this patient’s symptoms, what is the mechanism of action of the drug?

Inhibition of the formation of the translation initiation complex

Formation of free radical toxic metabolites that damage DNA

Inhibition of cell wall synthesis

Inhibition of DNA-dependent RNA polymerase

A 24-year-old woman presents to her physician's office complaining of a worsening cough with large volumes of mucoid sputum every morning and thick, foul-smelling sputum almost every time she coughs. She says that this cough started about one month ago and has been increasing in intensity. Over-the-counter medications are ineffective. Past medical history is significant for cystic fibrosis diagnosed at the age of 6 years, and pneumonia twice in the past 2 years. Other than a cough, she has no fever or any other concerns. A sputum sample grows aerobic, non-lactose fermenting, oxidase-positive, gram-negative bacillus. Which of the following treatment regimens is the most beneficial for her at this time?

Trimethoprim and sulfamethoxazole

Amoxicillin and clavulanic acid

A 61-year-old woman who recently emigrated from India comes to the physician because of a 2-month history of fever, fatigue, night sweats, and a productive cough. She has had a 5-kg (11-lb) weight loss during this period. She has a history of type 2 diabetes mellitus and poorly controlled asthma. She has had multiple asthma exacerbations in the past year that were treated with glucocorticoids. An x-ray of the chest shows a cavitary lesion of the posterior apical segment of the left upper lobe with consolidation of the surrounding parenchyma. The pathogen identified on sputum culture is found to be resistant to multiple drugs, including streptomycin. Which of the following mechanisms is most likely involved in bacterial resistance to this drug?

Alteration in 30S ribosomal subunit

Alteration in the sequence of gyrA genes

Upregulation of arabinosyl transferase production

Upregulation of mycolic acid synthesis

Inhibition of bacterial synthesis of RNA

A 23-year-old woman presents to your office for a gynecological exam. She says that she has been in good health and has no complaints. She has been in a steady monogamous relationship for the past year. Physical examination was unremarkable. Screening tests are performed and return positive for gonorrhea. You treat her with an intramuscular injection of ceftriaxone and 7 day course of doxycycline. What else is recommended for this case?

Treat her partner for gonorrhea and chlamydia

Perform an abdominal ultrasonography in order to rule out pelvic inflammatory disease

Recheck her in 1 week for gonorrhea and chlamydia

Treatment with penicillin G for potential co-infection with syphilis

Inform her that her partner is likely cheating on her

An investigator studying fungal growth isolates organisms from an infant with diaper rash. The isolate is cultured and exposed to increasing concentrations of nystatin. Selected colonies continue to grow and replicate even at high concentrations of the drug. Which of the following is the most likely explanation for this finding?

Reduced ergosterol content in cell membrane

Inactivation of cytosine permease

Altered binding site of squalene epoxidase

Expression of dysfunctional cytochrome P-450 enzymes

Fluoroquinolone resistance for USMLE Step 2 CK: High-Yield Microbiology Lessons

Fluoroquinolone MOA - Bacterial DNA Unravelers

Fluoroquinolones are bactericidal antibiotics that inhibit bacterial DNA synthesis.
They target key topoisomerase enzymes:
- DNA Gyrase (Topoisomerase II): The primary target in Gram-negative bacteria. It relieves torsional strain during DNA unwinding.
- Topoisomerase IV: The primary target in Gram-positive bacteria. It separates interlinked daughter DNA strands after replication.
Inhibition of these enzymes leads to irreversible DNA strand breaks, causing rapid cell death.

⭐ Fluoroquinolones exhibit concentration-dependent killing; higher drug levels result in a more rapid bactericidal effect.

Resistance Mechanisms - The Great Bacterial Escape

Target Site Mutations (Most Common)
- Alterations in DNA gyrase (gyrA) and topoisomerase IV (parC) genes.
- Mutations cluster in the quinolone resistance-determining region (QRDR).
- Stepwise accumulation of mutations leads to ↑ resistance levels.
Reduced Intracellular Concentration
- Efflux Pumps: Membrane proteins (e.g., NorA in S. aureus, AcrAB-TolC in E. coli) actively transport fluoroquinolones out of the cell.
- Decreased Permeability: Changes in porin proteins in Gram-negative bacteria limit drug entry.
Plasmid-Mediated Resistance
- Qnr Proteins: Shield DNA gyrase and topoisomerase IV from fluoroquinolone binding.
- Enzymatic Modification: Acetyltransferase AAC(6')-Ib-cr modifies and inactivates ciprofloxacin.

⭐ A single mutation in gyrA can confer low-level resistance, but high-level resistance typically requires additional mutations in gyrA and/or parC.

Bacterial antibiotic resistance mechanisms

Clinical Impact - When Cipro Can't Cope

Key Resistant Organisms:
- Neisseria gonorrhoeae (widespread resistance)
- Pseudomonas aeruginosa (hospital-acquired)
- MRSA & Campylobacter jejuni
- Enterobacteriaceae (e.g., E. coli causing resistant UTIs)
Clinical Consequences:
- Treatment Failure: Leads to prolonged illness in UTIs, pneumonia, and intra-abdominal infections.
- Shift to Alternatives: Forces use of broader-spectrum agents (e.g., carbapenems) or those with higher toxicity, increasing risks of side effects and promoting further resistance.

⭐ High resistance rates have made fluoroquinolones obsolete for treating gonorrhea; ceftriaxone is now the standard of care.

Detection & Strategy - Staying Ahead of Superbugs

Detection Methods
- Susceptibility Testing: Kirby-Bauer disk diffusion to assess zone of inhibition; Minimum Inhibitory Concentration (MIC) determination for quantitative measure.
- Molecular Assays: PCR to detect specific resistance genes like gyrA and parC mutations.
Prevention: Antimicrobial Stewardship
- Prescribe fluoroquinolones only when necessary, based on culture and sensitivity results.
- Avoid use for mild infections (e.g., uncomplicated cystitis) where other agents are effective.

⭐ Fluoroquinolone use is a significant risk factor for subsequent Clostridioides difficile infection (CDI).

Antibiogram: Fluoroquinolone Resistance in Common Pathogens

The primary mechanism of resistance is mutations in the quinolone resistance-determining regions (QRDRs) of genes encoding DNA gyrase (gyrA) and topoisomerase IV (parC).

These mutations alter the target enzymes, reducing the binding affinity of fluoroquinolones.

Plasmid-mediated resistance is another key mechanism, often involving Qnr proteins that protect DNA gyrase from the drug.

Efflux pumps can actively transport fluoroquinolones out of the bacterial cell, lowering intracellular drug concentrations.

Resistance is a major clinical concern for gram-negative pathogens like Pseudomonas aeruginosa and Neisseria gonorrhoeae.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play